During the fabrication of semiconductor devices using CMOS technology, conductive interconnects (or contacts) between different types of conductive material are required to form a circuit. For example, as an MOS transistor is formed in a silicon substrate's active area, its' gate, source and drain then have to be connected to other circuitry by a conductor, such as a conductively doped polysilicon line.
These interconnects are typically formed by depositing a layer of polysilicon over a desired buried contact location thus allowing the polysilicon to fill the contact hole and make physical connection to the underlying active area. Next the polysilicon is doped by a doping source to make an electrical connection. However, as critical dimensions became smaller, which shrinks the contact opening, a low resistive contact becomes very difficult if not impossible to obtain.
Several methods to form low resistive contact structures in MOS devices have been investigated that include: self-aligned silicides on the source/drain regions, elevated source/drain regions, buried-oxide MOS contacts and selectively deposited layers of metal in the contact holes. The most attractive approach turns out to be the self-aligned silicides on the source/drain regions as this self-aligned process does not entail any additional masking steps over a conventional contact formation process.
A typical process for forming self-aligned silicide contacts to an MOS transistor is shown in FIGS. 1a-1e. FIG. 1a shows previously doped active areas 2 (see FIG. 1a) implanted into silicon substrate 1 to form the transistor's source and drain. Oxide 3, separates substrate 1 from a polysilicon gate structure 4 to form the transistor's gate oxide.
In FIG. 1b, oxide 3 has been etched and silicon dioxide spacers 5 are deposited and etched. In FIGS. 1c and 1d, a metal has been deposited and annealed to form metal silicide 6, followed by selectively removing the unreacted metal.
Finally, in FIG. 1e, dielectric 7 is deposited and contact holes are opened down to silicide 6. Now metal 8 is deposited into the contact holes to make contact with silicide 6.
The typical approach to develop self-aligned silicide contacts provides good ohmic contacts between a source/drain active area and an upper lying conductor. Unfortunately, the upper lying conductor must be formed from a metal while polysilicon cannot be used as effectively without reintroducing the problems of a high resistive contact of conventional buried contacts.
The present invention introduces a process that further develops the use of silicide contacts and in particular self aligned silicide (or salicide) contacts, by developing a doping scheme that will allow the use of an upper layer of polysilicon as the interconnect layer between a silicided conductor while maintaining a low resistive contact.